Method and apparatus for beam failure recovery in communication system

ABSTRACT

Disclosed are a method and an apparatus for beam failure recovery in a communication system. The method for beam failure recovery includes the steps of: searching for a plurality of candidate beams when a beam failure is detected; transmitting a beam failure recovery request signal to a base station by using beam #1 of the plurality of candidate beams; receiving a beam failure recovery response signal in response to the beam failure recovery request signal via beam #1 from the base station; and transmitting an SR requesting a resource for transmission of multi-beam setting information to the base station, wherein the multi-beam setting information includes indexes of one or more beams excluding beam #1 of the plurality of candidate beams. Therefore, performance of the communication system can be improved.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 16/977,750, filed on Sep. 2, 2020, which is a U.S. NationalPhase entry from International Application No. PCT/KR2019/001802, filedFeb. 14, 2019, which claims priority to Korean Patent Application Nos.10-2018-0028699, filed Mar. 12, 2018, and 10-2018-0033158, Mar. 22,2018, the disclosure of which is incorporated by reference herein intheir entirety.

BACKGROUND 1. Field of the Invention

The present invention relates to a technique for beam failure recovery,and more particularly, to a technique for beam failure recovery for oneor more beams in a communication system.

2. Description of Related Art

With the development of information and communication technology,various wireless communication technologies are being developed. Typicalwireless communication technologies include long term evolution (LTE),new radio (NR), etc. defined in the 3^(rd) generation partnershipproject (3GPP) standard. The LTE may be one of fourth generation (4G)wireless communication technologies, and the NR may be one of fifthgeneration (5G) wireless communication technologies.

The 5G communication system (hereinafter, a NR communication system)using a higher frequency band (e.g., a frequency band of 6 GHz orhigher) than a frequency band (e.g., a frequency band lower below 6 GHz)of the 4G communication system is being considered for processing ofsoaring wireless data after commercialization of the 4G communicationsystem (e.g., communication system supporting the LTE). The 5Gcommunication system may support enhanced Mobile BroadBand (eMBB), UltraReliable Low Latency Communication (URLLC), massive Machine TypeCommunication (mMTC), and the like.

In the 5G communication system, a communication node (e.g., a basestation and a terminal) may perform communications in a beam-formingscheme using one or more beams. A beam failure may occur according to achange of a surrounding environment of the communication node, and abeam failure recovery procedure is introduced in the 5G communicationsystem to solve this problem. The beam failure recovery procedure mayinclude a beam failure detection step, a new candidate beamidentification step, a beam failure recovery request step, and a beamfailure recovery response step. However, detailed operations in thesteps for the beam failure recovery procedure are not clearly defined.In particular, a beam failure recovery procedure for a plurality ofbeams is not yet clearly defined.

SUMMARY

The present invention is directed to providing a method and an apparatusfor beam failure recovery for one or more beams in a communicationsystem.

A beam failure recovery method, according to a first embodiment of thepresent invention for achieving the above-described objective, maycomprise searching for a plurality of candidate beams when a beamfailure is detected; transmitting a beam failure recovery request signalto a base station using a beam #1 among the plurality of candidatebeams; receiving from the base station a beam failure recovery responsesignal that is a response to the beam failure recovery request signalthrough the beam #1; transmitting an SR to the base station, the SRrequesting a resource for transmission of multi-beam configurationinformation including index(es) of one or more beams excluding the beam#1 among the plurality of candidate beams; receiving a UL grant from thebase station, the uplink grant being a response to the SR; andtransmitting the multi-beam configuration information to the basestation using a resource indicated by the UL grant.

Here, the beam failure may be detected based on a measurement result ofa first reference signal configured by the base station, and the firstreference signal may have a QCL relationship with a DM-RS of a controlchannel between the base station and the terminal.

Here, the beam failure may be determined to be detected when error ratesof all beams used for transmitting and receiving a control channelbetween the terminal and the base station are larger than apreconfigured threshold, and when a part of all the beams are receivedwithin a monitoring period and error rates of the part of all the beamsare larger than the preconfigured threshold, a PHY entity of theterminal may transmit to a MAC entity of the terminal a flag indicatingresetting of a timer without increasing a counter indicating a number ofoccurrences of beam failure instances.

Here, the beam failure may be determined to be detected when error ratesof all beams used for transmitting and receiving a control channelbetween the terminal and the base station are larger than apreconfigured threshold, and when at least one beam among all the beamsis received within a monitoring period, a PHY entity of the terminal maytransmit to a MAC entity of the terminal information indicating abeam(s) having an error rate larger than the preconfigured threshold.

Here, the plurality of candidate beams may be searched based onmeasurement results of second reference signals configured by the basestation, and each of the second reference signals may have a QCLrelationship with a DM-RS of a control channel between the base stationand the terminal.

Here, the beam failure recovery request signal may be transmittedthrough a PRACH associated with the second reference signal.

Here, the beam failure recovery response signal may be received througha CORESET configured by the base station.

Here, the multi-beam configuration information may further include atleast one of information indicating that multiple beams can beconfigured and information of qualities of the one or more beams.

Here, the multi-beam configuration information may be transmitted to thebase station through a MAC CE.

A beam failure recovery method, according to a second embodiment of thepresent invention for achieving the above-described objective, maycomprise searching for a plurality of candidate beams when a beamfailure is detected; transmitting a beam failure recovery request signalto a base station using a beam #1 among the plurality of candidatebeams; receiving from the base station a beam failure recovery responsesignal including a UL grant through the beam #1; and transmitting to thebase station multi-beam configuration information including index(es) ofone or more beams excluding the beam #1 among the plurality of candidatebeams, by using a resource indicated by the UL grant.

Here, the method may further comprise, after receiving the beam failurerecovery response signal, receiving a medium access control (MAC)control element (CE) requesting reporting of the multi-beamconfiguration information from the base station, wherein the multi-beamconfiguration information is transmitted when the MAC CE is received.

Here, the beam failure may be detected based on a measurement result ofa first reference signal configured by the base station, and the firstreference signal may have a QCL relationship with a DM-RS of a controlchannel between the base station and the terminal.

Here, the plurality of candidate beams may be searched based onmeasurement results of second reference signals configured by the basestation, and each of the second reference signals may have a QCLrelationship with a DM-RS of a control channel between the base stationand the terminal.

Here, the multi-beam configuration information may further include atleast one of information indicating that multiple beams can beconfigured and information of qualities of the one or more beams.

Here, the multi-beam configuration information may be transmitted to thebase station together with a BSR.

A beam failure recovery method, according to a third embodiment of thepresent invention for achieving the above-described objective, maycomprise searching for a plurality of candidate beams when a beamfailure is detected; transmitting a first beam failure recovery requestsignal to a base station using a beam #1 among the plurality ofcandidate beams; receiving from the base station a first beam failurerecovery response signal that is a response to the first beam failurerecovery request signal through the beam #1; transmitting a second beamfailure recovery request signal to the base station using a beam #2among the plurality of candidate beams; and receiving from the basestation a second beam failure recovery response signal that is aresponse to the second beam failure recovery request signal through thebeam #2.

Here, the beam #1 may be searched based on a measurement result of afirst reference signal configured by the base station, the firstreference signal may have a QCL relationship with a DM-RS for a controlchannel between the base station and the terminal, and the first beamfailure recovery request signal may be transmitted through a first PRACHassociated with the first reference signal.

Here, the first PRACH may be configured for the beam #1 and a beam #3among the plurality of candidate beams, and the first beam failurerecovery request signal may be a signal for requesting recovery of thebeam #1 and the beam #3 which are associated with the first PRACH.

Here, the first PRACH may be configured independently of a second PRACHthrough which the second beam failure recovery request signal.

Here, configuration information of a PRACH used for recovery of beamfailures of two or more beams among the plurality of candidate beams maybe received from the base station through an RRC message.

Advantageous Effects

According to the present invention, when a beam failure instance occursin a beam among a plurality of beams, a physical (PHY) layer entity of aterminal may transmit to a medium access control (MAC) layer entity ofthe terminal a flag indicating resetting of a timer without increasing acounter or information indicating the beam in which the beam failureinstance has occurred. In this case, the MAC layer entity of theterminal may reset the timer without increasing the counter based on theinformation received from the PHY layer entity. Accordingly, even when aplurality of beams are used between the base station and the terminal, abeam failure can be accurately detected.

Further, since a plurality of beams can be recovered in the beam failurerecovery procedure, a separate configuration procedure for configuringthe plurality of beams may not be performed. Therefore, a plurality ofbeams can be quickly configured between the base station and theterminal, and the performance of the communication system can beimproved.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a conceptual diagram illustrating a first embodiment of acommunication system.

FIG. 2 is a block diagram illustrating a first embodiment of acommunication node constituting a communication system.

FIG. 3 is a timing diagram illustrating a first embodiment of a beamfailure recovery procedure in a communication system.

FIG. 4 is a timing diagram illustrating a first embodiment of a beamfailure detection step in a communication system.

FIG. 5 is a timing diagram illustrating a first embodiment of a beamfailure recovery response step in a communication system.

FIG. 6 is a timing diagram illustrating a first embodiment of amulti-beam recovery procedure in a communication system.

FIG. 7 is a timing diagram illustrating a second embodiment of amulti-beam recovery procedure in a communication system.

FIG. 8 is a timing diagram illustrating a third embodiment of amulti-beam recovery procedure in a communication system.

FIG. 9 is a timing diagram illustrating a fourth embodiment of amulti-beam recovery procedure in a communication system.

FIG. 10 is a timing diagram illustrating a fifth embodiment of amulti-beam recovery procedure in a communication system.

FIG. 11 is a timing diagram illustrating a sixth embodiment of amulti-beam recovery procedure in a communication system.

DETAILED DESCRIPTION OF THE INVENTION

While the present invention is susceptible to various modifications andalternative forms, specific embodiments are shown by way of example inthe drawings and described in detail. It should be understood, however,that the description is not intended to limit the present invention tothe specific embodiments, but, on the contrary, the present invention isto cover all modifications, equivalents, and alternatives that fallwithin the spirit and scope of the present invention.

Although the terms “first,” “second,” etc. may be used herein inreference to various elements, such elements should not be construed aslimited by these terms. These terms are only used to distinguish oneelement from another. For example, a first element could be termed asecond element, and a second element could be termed a first element,without departing from the scope of the present invention. The term“and/or” includes any and all combinations of one or more of theassociated listed items.

It will be understood that when an element is referred to as being“connected” or “coupled” to another element, it can be directlyconnected or coupled to the other element or intervening elements may bepresent. In contrast, when an element is referred to as being “directlyconnected” or “directed coupled” to another element, there are nointervening elements.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of embodiments ofthe present invention. As used herein, the singular forms “a,” “an,” and“the” are intended to include the plural forms as well, unless thecontext clearly indicates otherwise. It will be further understood thatthe terms “comprises,” “comprising,” “includes,” and/or “including,”when used herein, specify the presence of stated features, integers,steps, operations, elements, parts, and/or combinations thereof, but donot preclude the presence or addition of one or more other features,integers, steps, operations, elements, parts, and/or combinationsthereof.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by thoseof ordinary skill in the art to which the present invention pertains. Itwill be further understood that terms defined in commonly useddictionaries should be interpreted as having a meaning that isconsistent with their meaning in the context of the related art and willnot be interpreted in an idealized or overly formal sense unlessexpressly so defined herein.

Hereinafter, exemplary embodiments of the present invention will bedescribed in greater detail with reference to the accompanying drawings.To facilitate overall understanding of the present invention, likenumbers refer to like elements throughout the description of thedrawings, and description of the same component will not be reiterated.

A communication system to which embodiments according to the presentdisclosure will be described. However, the communication systems towhich embodiments according to the present disclosure are applied arenot restricted to what will be described below. That is, the embodimentsaccording to the present disclosure may be applied to variouscommunication systems. Here, the term ‘communication system’ may be usedwith the same meaning as the term ‘communication network’.

FIG. 1 is a conceptual diagram illustrating a first embodiment of acommunication system.

Referring to FIG. 1 , a communication system 100 may comprise aplurality of communication nodes 110-1, 110-2, 110-3, 120-1, 120-2,130-1, 130-2, 130-3, 130-4, 130-5, and 130-6. The plurality ofcommunication nodes may support 4G communication (e.g., long termevolution (LTE), LTE-advanced (LTE-A)), 5G communication (e.g., newradio (NR)), or the like. The 4G communication may be performed in afrequency band below 6 GHz, and the 5G communication may be performed ina frequency band above 6 GHz as well as the frequency band below 6 GHz.

For example, for the 4G communication and the 5G communication, theplurality of communication nodes may support code division multipleaccess (CDMA) technology, wideband CDMA (WCDMA) technology, timedivision multiple access (TDMA) technology, frequency division multipleaccess (FDMA) technology, orthogonal frequency division multiplexing(OFDM) technology, filtered OFDM technology, cyclic prefix OFDM(CP-OFDM) technology, discrete Fourier transform-spread-OFDM(DFT-s-OFDM) technology, single carrier FDMA (SC-FDMA) technology,non-orthogonal multiple access (NOMA) technology, generalized frequencydivision multiplexing (GFDM) technology, filter band multi-carrier(FBMC) technology, universal filtered multi-carrier (UFMC) technology,space division multiple access (SDMA) technology, or the like.

Also, the communication system 100 may further comprise a core network.When the communication system supports the 4G communication, the corenetwork may include a serving gateway (S-GW), a packet data network(PDN) gateway (P-GW), a mobility management entity (MME), and the like.When the communication system 100 supports the 5G communication, thecore network may include an access and mobility management function(AMF), a user plane function (UPF), a session management function (SMF),and the like.

Meanwhile each of the plurality of communication nodes 110-1, 110-2,110-3, 120-1, 120-2, 130-1, 130-2, 130-3, 130-4, 130-5, and 130-6constituting the communication system 100 may have the followingstructure.

FIG. 2 is a block diagram illustrating a first embodiment of acommunication node constituting a communication system.

Referring to FIG. 2 , a communication node 200 may comprise at least oneprocessor 210, a memory 220, and a transceiver 230 connected to thenetwork for performing communications. Also, the communication node 200may further comprise an input interface device 240, an output interfacedevice 250, a storage device 260, and the like. Each component includedin the communication node 200 may communicate with each other asconnected through a bus 270.

However, each component included in the communication node 200 may notbe connected to the common bus 270 but may be connected to the processor210 via an individual interface or a separate bus. For example, theprocessor 210 may be connected to at least one of the memory 220, thetransceiver 230, the input interface device 240, the output interfacedevice 250 and the storage device 260 via a dedicated interface.

The processor 210 may execute a program stored in at least one of thememory 220 and the storage device 260. The processor 210 may refer to acentral processing unit (CPU), a graphics processing unit (GPU), or adedicated processor on which methods in accordance with embodiments ofthe present disclosure are performed. Each of the memory 220 and thestorage device 260 may be constituted by at least one of a volatilestorage medium and a non-volatile storage medium. For example, thememory 220 may comprise at least one of read-only memory (ROM) andrandom access memory (RAM).

Referring again to FIG. 1 , the communication system 100 may comprise aplurality of base stations 110-1, 110-2, 110-3, 120-1, and 120-2, and aplurality of terminals 130-1, 130-2, 130-3, 130-4, 130-5, and 130-6.Each of the first base station 110-1, the second base station 110-2, andthe third base station 110-3 may form a macro cell, and each of thefourth base station 120-1 and the fifth base station 120-2 may form asmall cell. The fourth base station 120-1, the third terminal 130-3, andthe fourth terminal 130-4 may belong to cell coverage of the first basestation 110-1. Also, the second terminal 130-2, the fourth terminal130-4, and the fifth terminal 130-5 may belong to cell coverage of thesecond base station 110-2. Also, the fifth base station 120-2, thefourth terminal 130-4, the fifth terminal 130-5, and the sixth terminal130-6 may belong to cell coverage of the third base station 110-3. Also,the first terminal 130-1 may belong to cell coverage of the fourth basestation 120-1, and the sixth terminal 130-6 may belong to cell coverageof the fifth base station 120-2.

Here, each of the plurality of base stations 110-1, 110-2, 110-3, 120-1,and 120-2 may refer to a Node-B, a evolved Node-B (eNB), a basetransceiver station (BTS), a radio base station, a radio transceiver, anaccess point, an access node, or the like. Also, each of the pluralityof terminals 130-1, 130-2, 130-3, 130-4, 130-5, and 130-6 may refer to auser equipment (UE), a terminal, an access terminal, a mobile terminal,a station, a subscriber station, a mobile station, a portable subscriberstation, a node, a device, or the like.

Meanwhile, each of the plurality of base stations 110-1, 110-2, 110-3,120-1, and 120-2 may operate in the same frequency band or in differentfrequency bands. The plurality of base stations 110-1, 110-2, 110-3,120-1, and 120-2 may be connected to each other via an ideal backhaul ora non-ideal backhaul, and exchange information with each other via theideal or non-ideal backhaul. Also, each of the plurality of basestations 110-1, 110-2, 110-3, 120-1, and 120-2 may be connected to thecore network through the ideal or non-ideal backhaul. Each of theplurality of base stations 110-1, 110-2, 110-3, 120-1, and 120-2 maytransmit a signal received from the core network to the correspondingterminal 130-1, 130-2, 130-3, 130-4, 130-5, or 130-6, and transmit asignal received from the corresponding terminal 130-1, 130-2, 130-3,130-4, 130-5, or 130-6 to the core network.

Also, each of the plurality of base stations 110-1, 110-2, 110-3, 120-1,and 120-2 may support a multi-input multi-output (MIMO) transmission(e.g., a single-user MIMO (SU-MIMO), a multi-user MIMO (MU-MIMO), amassive MIMO, or the like), a coordinated multipoint (CoMP)transmission, a carrier aggregation (CA) transmission, a transmission inunlicensed band, a device-to-device (D2D) communications (or, proximityservices (ProSe)), or the like. Here, each of the plurality of terminals130-1, 130-2, 130-3, 130-4, 130-5, and 130-6 may perform operationscorresponding to the operations of the plurality of base stations 110-1,110-2, 110-3, 120-1, and 120-2 (i.e., the operations supported by theplurality of base stations 110-1, 110-2, 110-3, 120-1, and 120-2). Forexample, the second base station 110-2 may transmit a signal to thefourth terminal 130-4 in the SU-MIMO manner, and the fourth terminal130-4 may receive the signal from the second base station 110-2 in theSU-MIMO manner. Alternatively, the second base station 110-2 maytransmit a signal to the fourth terminal 130-4 and fifth terminal 130-5in the MU-MIMO manner, and the fourth terminal 130-4 and fifth terminal130-5 may receive the signal from the second base station 110-2 in theMU-MIMO manner.

The first base station 110-1, the second base station 110-2, and thethird base station 110-3 may transmit a signal to the fourth terminal130-4 in the CoMP transmission manner, and the fourth terminal 130-4 mayreceive the signal from the first base station 110-1, the second basestation 110-2, and the third base station 110-3 in the CoMP manner.Also, each of the plurality of base stations 110-1, 110-2, 110-3, 120-1,and 120-2 may exchange signals with the corresponding terminals 130-1,130-2, 130-3, 130-4, 130-5, or 130-6 which belongs to its cell coveragein the CA manner. Each of the base stations 110-1, 110-2, and 110-3 maycontrol D2D communications between the fourth terminal 130-4 and thefifth terminal 130-5, and thus the fourth terminal 130-4 and the fifthterminal 130-5 may perform the D2D communications under control of thesecond base station 110-2 and the third base station 110-3.

Next, a beam failure recovery procedure in the communication system willbe described. Even when a method (e.g., transmission or reception of asignal) to be performed at a first communication node amongcommunication nodes is described, a corresponding second communicationnode may perform a method (e.g., reception or transmission of thesignal) corresponding to the method performed at the first communicationnode. That is, when an operation of a terminal is described, acorresponding base station may perform an operation corresponding to theoperation of the terminal. Conversely, when an operation of the basestation is described, the corresponding terminal may perform anoperation corresponding to the operation of the base station.

The embodiments below may be applied to other communication systems(e.g., 4G communication systems) as well as a 5G communication system.Also, in the embodiments below, physical channels and signals may beused in the 5G communication systems and other communication systems(e.g., 4G communication systems).

The communication system (e.g., 5G communication system) may operate ina high frequency band, and the diffraction characteristic and thereflection characteristic of the radio wave are poor in the highfrequency band. Thus, a propagation loss (e.g., path loss, reflectionloss, and the like) in the high frequency band may be larger than apropagation loss in a low frequency band. In this case, a cell coverageof a communication system supporting the high frequency band may besmaller than a cell coverage of a communication system supporting thelow frequency band. In order to solve such the problem, a beamformingscheme based on a plurality of antenna elements may be used to increasethe cell coverage in the communication system supporting the highfrequency band.

A beam management procedure may be introduced for communications basedon the beamforming scheme. The beam management procedure may include aninitial beam establishment procedure, a beam adjustment procedure, and abeam failure recovery procedure.

The communications between the base station and the terminal may beperformed using a beam. During the communications between the basestation and the terminal, the beam may be blocked due to rotation of theterminal, movement of the terminal, mismatch of directions of beamsbetween the base station and the terminal, or occurrence of an obstacle.In this case, a quality of a radio link between the base station and theterminal may be rapidly deteriorated. When the quality of the radio linkdegrades in a data channel (e.g., a physical downlink shared channel(PDSCH)), the terminal may perform the beam management procedure througha control channel (e.g., a physical downlink control channel (PDCCH)),and select a beam having a radio link of good quality. However, when thequality of the radio link degrades in a control channel, the terminalmay not receive a control signal from the base station. In this case,the communications between the base station and the terminal may becomeimpossible, and this situation may be defined as a ‘beam failure’.

In a communication system supporting multiple beams, a control channelmay be transmitted using multiple beams. A beam failure may occur in apart of the multiple beams. Alternatively, a beam failure may occur inall beams used for transmission of the control channel. A beam failureat some beams may be defined as a ‘partial beam failure’, and a ‘beamfailure’ may mean a beam failure at all beams used for transmission ofthe control channel.

The beam failure may mean that the radio link between the base stationand the terminal is disconnected, and a procedure for managing andrecovering the beam failure (e.g., beam failure recovery procedure) isneeded. The beam failure recovery procedure may be performed as follows.

FIG. 3 is a timing diagram illustrating a first embodiment of a beamfailure recovery procedure in a communication system.

Referring to FIG. 3 , a beam failure recovery procedure may beconfigured in four steps. For example, the beam failure recoveryprocedure may include a beam failure detection step, a new candidatebeam identification step, a beam failure recovery request step, and abeam failure recovery response step. The beam failure recovery proceduremay vary depending on a transmission scheme of a beam failure recoveryrequest signal. The beam failure recovery request signal may betransmitted using a contention-free random access (RA) scheme, acontention-based RA scheme, or a physical uplink control channel (PUCCH)based scheme.

The terminal may receive a reference signal (e.g., channel stateinformation reference signal (CSI-RS), demodulation-reference signal(DM-RS), a phase tracking RS (PT-RS), a cell-specific reference signal(CRS), etc.) or a synchronization signal/physical broadcast channel(SS/PBCH) block, and may periodically check whether a beam failure hasoccurred based on the reference signal or the SS/PBCH block. Also, theterminal may search for new candidate beam(s) when the beam failure isdetected. The new candidate beam identification step may be performedafter or before the beam failure detection step. Alternatively, the newcandidate beam identification step may be performed together with thebeam failure detection step.

The terminal may transmit a beam failure recovery request signal to thebase station using the beam (i.e., the searched beam) identified in thenew candidate beam identification step, and may receive a beam failurerecovery response signal, which is a response to the beam failurerecovery request signal, from the base station through the beamidentified in the new candidate beam identification step. Also, the beamidentified in the new candidate beam identification step may be used fortransmission and reception of control channels and/or data channelsuntil an optimal beam is configured.

When the beam failure is detected and the new candidate beam isidentified, the terminal may transmit the beam failure recovery requestsignal to the base station through a contention-free physical randomaccess channel (PRACH), and may perform a monitoring operation in apreconfigured duration in order to receive the beam failure recoveryresponse signal. An interval between a transmission time point of thebeam failure recovery request signal and a starting time point of thepreconfigured period may be 4 slots. The base station may receive thebeam failure recovery request signal from the terminal, and may transmitthe beam failure recovery response signal to the terminal within thepreconfigured period.

When the beam failure recovery response signal is received from the basestation within the preconfigured period, the terminal may determine thatthe beam failure has been recovered. In this case, the terminal mayreceive control information from the base station through the beam usedfor transmitting the beam failure recovery request signal, and mayconfigure a new beam with the base station based on the received controlinformation. Alternatively, the terminal may perform datatransmission/reception procedures with the base station based onresource allocation information included in the received controlinformation.

Next, detailed operations of the communication node (e.g., base stationand terminal) in the beam failure detection step, the new candidate beamidentification step, the beam failure recovery request step, and thebeam failure recovery response step will be described.

▪ Beam Failure Detection Step

A reference signal used to detect a beam failure may be defined as abeam failure detection RS (BFD-RS). One or more reference signals amongCSI-RS, DM-RS, PT-RS, and CRS may be used as the BFD-RS. Alternatively,the SS/PBCH block may be used as the BFD-RS.

When the CSI-RS is used as the BFD-RS, the base station may transmit tothe terminal an RRC message including information indicating that CSI-RSresource(s) is to be used for the BFD-RS. The terminal may receive theRRC message from the base station, and may determine that the CSI-RSresource(s) configured by the base station is used for the BFD-RS basedon the information included in the RRC message. In this case, the basestation may use the same beam as a beam used for transmission of acontrol channel (e.g., DM-RS for the control channel) to transmit theCSI-RS through the CSI-RS resource(s) used for the BFD-RS. The terminalmay perform a monitoring operation to receive the CSI-RS through theCSI-RS resource(s) used for the BFD-RS by using the same beam as thebeam used for reception of the control channel (e.g., DM-RS for thecontrol channel).

Here, the CSI-RS resource(s) may have a spatial quasi co-location (QCL)relationship with the DM-RS for the control channel. The base stationmay transmit to the terminal downlink control information (DCI)including a transmission configuration indication (TCI) indicatinginformation on the CSI-RS resource(s) having a spatial QCL relationshipwith the DM-RS for the control channel. The terminal may receive the DCIfrom the base station, and identify the CSI-RS resource(s) having aspatial QCL relationship with the DM-RS for the control channel based onthe TCI included in the DCI.

That is, the CSI-RS resource(s) having a spatial QCL relationship withthe DM-RS for the control channel may be configured through at least oneof the RRC message and the DCI. When the RRC message indicating theCSI-RS resource(s) having a spatial QCL relationship with the DM-RS forthe control channel is not received, the terminal may transmit theCSI-RS or the SS/PBCH block indicated by the TCI included in the DCI asthe BFD-RS.

The terminal may measure a radio link quality of the control channelusing the BFD-RS (e.g., CSI-RS or SS/PBCH block configured as theBFD-RS). The radio link quality may be a hypothetical block error rate(BLER). For example, when the hypothetical BLER measured based on theBFD-RS is larger than a preset threshold value Q_(out_LR), the terminalmay determine that the radio link of the corresponding beam isdisconnected. The threshold Q_(out_LR) may be an out-of-synchronizationthreshold used in radio link monitoring (RLM).

The beam may be configured for each control resource set (CORESET). Forexample, one beam for one CORESET may be configured. If multipleCORESETs are configured, the base station may transmit a control channelusing multiple beams. If the control channel is transmitted usingmultiple beams, the terminal may determine that a beam failure hasoccurred when the radio link qualities for all the beams used fortransmission of the control channel is greater than the thresholdQ_(out_LR).

However, since a complicated beam failure recovery procedure should beperformed in order to recover the beam failure, it may be inefficientfor the terminal to determine the occurrence of the beam failure basedon one measurement result. Thus, the beam failure may be determinedbased on several measurement results. When a result of one measurementfor the BFD-RS is larger than the threshold value Q_(out_LR), theterminal may determine that one beam failure instance has occurred. Theoccurrence of the beam failure instance may be determined by a physical(PHY) layer (e.g., a PHY entity performing a PHY layer function) or amedium access control (MAC) layer (e.g., a MAC layer entity performing aMAC layer function) belonging to the communication node. The MAC layermay determine that a beam failure has occurred when beam failureinstances have occurred more than a preconfigured number of times.

In order for the MAC layer to determine the beam failure, the PHY layerbelonging to the communication node may recognize whether the beamfailure instance has occurred or not based on the measurement result ofthe radio link quality, and report to the MAC layer information relatedto the beam failure instance (e.g., information indicating that the beamfailure instance has occurred, the measurement result of the radio linkquality, and an index of the beam). The information related to the beamfailure instance may be referred to as ‘instance information’. Theinstance information may be periodically reported from the PHY layer tothe MAC layer. For example, when it is determined that the beam failureinstance has occurred, the PHY layer may transmit the instanceinformation to the MAC layer. When it is determined that the beamfailure instance has not occurred, the PHY layer may not transmit theinstance information to the MAC layer.

The MAC layer may have a timer and a counter to determine the occurrenceof the beam failure. The timer may be used to reset the counter, and thecounter may be used to calculate the number of consecutive beam failureinstances. When the timer expires, the counter may be reset. In thiscase, the counter may be set to ‘0’. The MAC layer may reset the timerand increment the counter whenever the instance information is receivedfrom the PHY layer. When the timer is reset, the timer may be set to aninitial value. The MAC layer may determine that a beam failure hasoccurred when the incremented counter reaches a preconfigured value.

The value used for determining the beam failure may be preconfigured bythe base station, and the base station may transmit the preconfiguredvalue using one or more of an RRC message, a MAC control element (CE),and a DCI. Alternatively, the value used for determining the beamfailure may be preconfigured to a fixed value between the base stationand the terminal. When the timer expires without receiving the instanceinformation, the MAC layer may reset the counter. The length of thetimer may be determined based on a reporting periodicity of the instanceinformation.

On the other hand, when multiple beams are used, a transmission periodof the BFD-RS for each of the beams may be different. In this case, thefollowing problems may occur in the beam failure detection step.

FIG. 4 is a timing diagram illustrating a first embodiment of a beamfailure detection step in a communication system.

Referring to FIG. 4 , when a control channel is transmitted using twobeams (e.g., beam #1 and beam #2), two BFD-RSs (e.g., BFD-RS #1 andBFD-RS #2) for the two beams may be configured. In this case, the basestation may transmit the BFD-RS #1 using the beam #1 and the BFD-RS #2using the beam #2. A transmission periodicity of the BFD-RS #1 may bedifferent from a transmission periodicity of the BFD-RS #2. Here, whenmeasurement results (e.g., error rates) of the BFD-RSs (e.g., BFD-RS #1and BFD-RS #2) of all the beams (e.g., beam #1 and beam #2) are greaterthan the threshold value Q_(out_LR), the PHY layer may determine that abeam failure instance has occurred, and may report the instanceinformation to the MAC layer.

However, since the transmission periodicity of the BFD-RS #1 isdifferent from the transmission periodicity of the BFD-RS #2, the PHYlayer may not receive all the BFD-RSs in a specific reporting period(e.g., reporting period #2 and reporting period #4). Therefore, evenwhen the measurement result (e.g., error rate) of one received BFD-RS(e.g., BFD-RS #1) in a specific reporting period is greater than thethreshold value Q_(out_LR), the PHY layer may determine that a beamfailure instance has not occurred.

In this case, the instance information may not be reported from the PHYlayer to the MAC layer in the specific reporting period. Also, the MAClayer may reset the counter when the timer expires at the specificreporting period, since the instance information is not received fromthe PHY layer. That is, although a beam failure has actually occurred,the MAC layer may not determine that the beam failure has occurred. Inorder to solve such the problem, the PHY layer and MAC layer may operateas follows.

√ Scheme 1

When measurement results (e.g., error rates) of all the BFD-RSs (e.g.,BFD-RS #1 and BFD-RS #2) received in a reporting period are greater thanthe threshold value Q_(out_LR), the PHY layer may determine that a beamfailure instance has occurred, and may report the instance informationto the MAC layer. When only a part (e.g., BFD-RS #1) of the BFD-RSs isreceived in a specific reporting period (e.g., reporting period #2 andreporting period #4) and the measurement result (e.g., error rate) ofthe BFD-RS #1 is greater than the threshold value Q_(out_LR), the PHYlayer may determine that a partial beam failure instance has occurred.In this case, the PHY layer may report the instance information to theMAC layer together with a flag indicating resetting of the timer withoutincreasing the counter. When the flag and instance information arereceived from the PHY layer, the MAC layer may determine that a partialbeam failure instance has occurred, and may reset the timer withoutincreasing the counter based on the flag.

√ Scheme 2

The instance information reported from the PHY layer to the MAC layermay further include information indicating a BFD-RS having a BLER largerthan the threshold value Q_(out_LR) (e.g., a beam index or TCIindicating a beam through which the BFD-RS is transmitted).

For example, when measurement results of all the BFD-RSs (e.g., BFD-RS#1 and BFD-RS #2) received in a reporting period #1 are greater than thethreshold value Q_(out_LR), the PHY layer may report to the MAC layerinstance information including information indicating the BFD-RS #1 andinformation indicating the BFD-RS #2. Alternatively, the PHY layer mayreport instance information #1 including information indicating BFD-RS#1 and instance information #2 including information indicating BFD-RS#2 to the MAC layer. The MAC layer may receive the instance information(or, the instance information #1 and the instance information #2) fromthe PHY layer, and may determine that a beam failure instance hasoccurred based on the instance information (or, the instance information#1 and the instance information #2). In this case, the MAC layer mayincrease the counter and reset the timer.

For example, when a measurement result (e.g., error rate) of a part(e.g., BFD-RS #1) of the BFD-RSs received in a reporting period #2 isgreater than the threshold value Q_(out_LR), the PHY layer may report tothe MAC layer instance information including information indicating theBFD-RS #1. The MAC layer may receive the instance information from thePHY layer and may determine that a partial beam failure instance hasoccurred based on the information included in the instance information.In this case, the MAC layer may reset the timer without increasing thecounter.

√ Scheme 3

The base station may adjust the length of a reporting period so that theradio link qualities can be measured based on all the BFD-RSs in onereporting period. The length of the reporting period may be set to belonger than the longest transmission periodicity among the transmissionperiodicities of all the BFD-RSs. The base station may transmitinformation indicating the configured reporting period using one or moreof RRC message, MAC CE, and DCI. The terminal may confirm the reportingperiod configured by the base station by receiving the RRC message, MACCE, or DCI, and measure the radio link quality based on the receivedBFD-RS in the report period.

▪ New Candidate Beam Identification Step

The new candidate beam identification step may be performed concurrentlywith the beam failure detection step. When a beam failure is detectedand a new candidate beam(s) is identified, the terminal may transmit abeam failure recovery request signal. Therefore, in order to search fora new candidate beam(s) when a beam failure occurs, the terminal mayreceive reference signals and/or SS/PBCH blocks from the base station.

The reference signal used to detect the new candidate beam may bedefined as a new beam identification (NBI) RS (NBI-RS). One or morereference signals of CSI-RS, DM-RS, PT-RS, and CRS may be used as theNBI-RS. Alternatively, the SS/PBCH block may be used as the NBI-RS.

When the CSI-RS is used as the NBI-RS, the base station may transmit tothe terminal an RRC message including information indicating that CSI-RSresource(s) is to be used for the NBI-RS. The terminal may receive theRRC message from the base station, and may determine that the CSI-RSresource(s) configured by the base station is used for the NBI-RS basedon the information included in the RRC message. Alternatively, when theSS/PBCH block is used as the NBI-RS, the base station may transmit anRRC message to the terminal, which includes information indicating thatthe SS/PBCH block is to be used for the NBI-RS. The terminal may receivethe RRC message from the base station, and may determine that theSS/PBCH block is to be used for the NBI-RS based on the informationincluded in the RRC message.

The NBI-RS may be transmitted through a candidate beam(s) used fortransmission and reception of a data channel as well as a controlchannel. The NBI-RS may have a spatial QCL relationship with a DM-RS fora control channel or a data channel (e.g., a data channel before beamreconfiguration). In this case, the base station may use the same beamas that used for transmission of a control channel (e.g. DM-RS for thecontrol channel) or a data channel (e.g. DM-RS for the data channel) totransmit the NBI-RS. The terminal may use the same beam as that used forthe reception of a control channel (e.g. DM-RS for the control channel)or a data channel (e.g., DM-RS for the data channel) to perform amonitoring operation for reception of the NBI-RS.

The terminal may receive the NBI-RS, and measure the quality of the beambased on the NBI-RS. The quality of the beam may be an L1-referencesignal received power (L1-RSRP). The L1-RSRP may be an RSRP measured atthe PHY layer. For example, when the L1-RSRP measured based on theNBI-RS is larger than a preconfigured threshold value Q_(in_LR), theterminal may determine the beam used for transmission of thecorresponding NBI-RS as a new candidate beam. The threshold valueQ_(in_LR) may be transmitted from the base station to the terminalthrough an RRC message. When a plurality of beams having an L1-RSRPlarger than the threshold value Q_(in_LR) are identified, the MAC layermay determine one or more beams among the plurality of identified beamsas new candidate beam(s).

Therefore, the PHY layer may configure a set of a plurality of beamshaving an L1-RSRP larger than the threshold value Q_(in_LR) (hereinafterreferred to as a ‘candidate beam set’). The candidate beam set mayinclude all or some of the beams having an L1-RSRP greater than thethreshold Q_(in_LR). The PHY layer may transmit indexes of therespective beams belonging to the candidate beam set (or, indexes of therespective NBI-RSs transmitted through the beams belonging to thecandidate beam set) and/or the L1-RSRPs of the respective beamsbelonging to the candidate beam set to the MAC Layer. The MAC layer mayreceive the indexes of the respective beams belonging to the candidatebeam set (or, indexes of the respective NBI-RSs transmitted through thebeams belonging to the candidate beam set) and/or the L1-RSRPs of therespective beams belonging to the candidate beam set, and based on thereceived information, the MAC layer may determine one or more beamsamong the beams belonging to the candidate beam set as the new candidatebeam(s).

▪ Beam Failure Recovery Request Step

When a beam failure is detected and at least one new candidate beam isidentified, the terminal may declare a beam failure. The terminal maytransmit a beam failure recovery request signal after declaring the beamfailure. The beam failure recovery request signal may be transmittedaccording to a recovery timer and the maximum number of recoveryrequests. The recovery timer and the maximum number of recovery requestsmay be configured by the base station, and the base station may transmitan RRC message to the terminal, including the recovery timer and themaximum number of recovery requests. The terminal may confirm therecovery timer and the maximum number of recovery requests by receivingthe RRC message. For example, when a beam failure is declared, therecovery timer may operate and the terminal may transmit a beam failurerecovery request signal to the base station.

In this case, the terminal may transmit the beam failure recoveryrequest signal to the base station through a PRACH by using the newcandidate beam. The beam failure recovery request signal may betransmitted based on the contention-free RA scheme. The terminal mayretransmit the beam failure recovery request signal until the terminalreceives a beam failure recovery response signal, which is a response tothe beam failure recovery request signal, from the base station.

A PRACH resource (e.g., a time-frequency resource of the PRACH, preambleID) used for transmission of the beam failure recovery request signalmay be associated with the new candidate beam (e.g., NBI-RS transmittedthrough the new candidate beam). The base station may transmitinformation on the PRACH resource associated with the NBI-RS to theterminal through an RRC message. The terminal may confirm theinformation on the PRACH resource associated with the NBI-RS byreceiving the RRC message, and may transmit the beam failure recoveryrequest signal based on the information on the PRACH resource.

▪ Beam Failure Recovery Response Step

When the beam failure recovery request signal is received from theterminal, the base station may transmit a beam failure recovery responsesignal to the terminal, which is a response to the beam failure recoveryrequest signal. The beam failure recovery response signal may betransmitted through a preconfigured CORESET. The CORESET used fortransmission of the beam failure recovery response signal may bereferred to as a ‘CORESET-beam failure recovery (CORESET-BFR)’. The beamused for transmission of the beam failure recovery response signal maybe the new candidate beam determined in the new candidate beamidentification step. That is, the beam used for transmission of the beamfailure recovery response signal may be associated with the beam usedfor receiving the beam failure recovery request signal.

FIG. 5 is a timing diagram illustrating a first embodiment of a beamfailure recovery response step in a communication system.

Referring to FIG. 5 , the terminal may transmit a beam failure recoveryrequest signal in a slot #n after declaring a beam failure. n may be aninteger equal to or greater than 0. The terminal may perform amonitoring operation from a slot #(n+4) to receive a beam failurerecovery response signal, which is a response to the beam failurerecovery request signal. The length of the monitoring period for thebeam failure recovery response signal may be configured by the basestation and the base station may transmit information indicating themonitoring period for the beam failure recovery response signal throughan RRC message. The terminal may confirm the monitoring period for thebeam failure recovery response signal by receiving the RRC message fromthe base station.

When the beam failure recovery response signal is not received withinthe monitoring period, the terminal may retransmit the beam failurerecovery request signal. The retransmission procedure of the beamfailure recovery request signal may be performed until the expiration ofthe recovery timer. Also, if the number of transmissions of the beamfailure recovery request signal reaches the maximum number of recoveryrequests before the expiration of the recovery timer, the terminal maystop the retransmission procedure of the beam failure recovery requestsignal.

If the beam failure recovery response signal is not received beforeexpiration of the recovery timer, the terminal may determine that thebeam failure recovery procedure has failed. Also, if the number oftransmissions of the beam failure recovery request signal reaches themaximum number of recovery requests before the expiration of therecovery timer, the terminal may determine that the beam failurerecovery procedure has failed. When it is determined that the beamfailure recovery procedure has failed, a radio link failure recoveryprocedure may be performed. If the beam failure recovery response signalis received before expiration of the recovery timer, the beam failurerecovery procedure may be completed. In this case, the communicationnode (e.g., the base station and the terminal) may use the beam (e.g.,the beam determined in the new candidate beam identification step)recovered in the beam failure recovery procedure to perform transmissionand reception procedures of control channels and data channels untilconfiguration of a new beam.

▪ Multiple Beam Recovery Procedure

In the beam failure recovery procedure, one beam may be recovered, theterminal may receive a control channel from the base station through therecovered beam, and perform a beam management procedure based on theinformation included in the control channel so as to configure multiplebeams. In this case, it may take a lot of time to configure multiplebeams.

In the new candidate beam identification step, the terminal measures thequalities of the beams, so that the terminal is able to identify whethermultiple beams can be configured based on the measured qualities. Thus,when the terminal is able to report to the base station that themultiple beams can be configured, the base station may easily configurethe multiple beams. That is, even when beam management/reportinginformation for multi-beam configuration is not configured by the basestation, the terminal may report information for multi-beamconfiguration (hereinafter referred to as ‘multi-beam configurationinformation’) to the base station. The multi-beam configurationinformation may include information indicating that multiple beams canbe configured. Also, the multi-beam configuration information mayfurther include at least one of indexes of configurable beams andqualities (e.g., L1-RSRPs) of them. The procedure of reporting themulti-beam configuration information may be performed as follows.

FIG. 6 is a timing diagram illustrating a first embodiment of amulti-beam recovery procedure in a communication system.

Referring to FIG. 6 , when the beam failure recovery procedure iscompleted, the terminal may transmit to the base station a schedulingrequest (SR) for the multi-beam configuration information through anuplink channel (e.g., a PUCCH or a physical uplink shared channel(PUSCH)). Even when a beam failure occurs, since uplink connectionbetween the base station and the terminal can be maintained, the SR maybe transmitted through the uplink channel. Alternatively, when the beamfailure recovery response signal includes an UL grant, the SR for themultiple beam configuration information may be transmitted through anuplink channel indicated by the UL grant.

The base station may receive the SR from the terminal, and allocate aresource for uplink transmission based on the SR. The base station maytransmit the UL grant including resource allocation information to theterminal through a control channel (e.g., PDCCH). The UL grant may beincluded in a DCI. The terminal may receive the UL grant from the basestation, and may transmit the multi-beam configuration information(e.g., indexes and/or qualities of configurable beams) through theuplink channel (e.g., PUSCH) indicated by the UL grant. The multi-beamconfiguration information may be included in a MAC CE, and the MAC CEincluding the multi-beam configuration information may be transmittedthrough the PUSCH.

The base station may receive the multi-beam configuration informationfrom the terminal, and may determine that multiple beams can beconfigured based on the multi-beam configuration information. Thus,multiple beams for communications between the base station and theterminal may be configured.

FIG. 7 is a timing diagram illustrating a second embodiment of amulti-beam recovery procedure in a communication system.

Referring to FIG. 7 , in the beam failure recovery procedure, the basestation may transmit a beam failure recovery response signal includingan UL grant to the terminal. The terminal may receive the beam failurerecovery response signal from the base station and identify the UL grantincluded in the beam failure recovery response signal. The terminal maytransmit to the base station multi-beam configuration information (e.g.,indexes and/or qualities of configurable beams) together with a bufferstatus report (BSR) through an uplink channel (e.g., PUSCH) indicated bythe UL grant. The BSR and the multi-beam configuration information maybe included in a MAC CE, and the MAC CE including the BSR and multi-beamconfiguration information may be transmitted through the PUSCH.

The base station may receive the BSR and multi-beam configurationinformation from the terminal, and may determine that multiple beams canbe configured based on the multi-beam configuration information. Thus,multiple beams for communications between the base station and theterminal may be configured.

FIG. 8 is a timing diagram illustrating a third embodiment of amulti-beam recovery procedure in a communication system.

Referring to FIG. 8 , when an uplink channel (e.g., PUCCH or PUSCH) isnot configured in the beam failure recovery procedure or when an uplinkconnection between the base station and the terminal is released, aftercompleting the beam failure recovery procedure, the terminal mayestablish an uplink connection between the base station and the terminalby performing a random access procedure with the base station. Theterminal may transmit multi-beam configuration information (e.g.,indexes and/or qualities of configurable beams) to the base stationthrough an uplink channel (e.g., PUSCH) configured by the random accessprocedure. The multi-beam configuration information may be included in aMAC CE, and the MAC CE including the multi-beam configurationinformation may be transmitted through the PUSCH. Further, themulti-beam configuration information may be transmitted to the basestation together with the BSR.

The base station may receive the multi-beam configuration informationfrom the terminal, and may determine that multiple beams can beconfigured based on the multi-beam configuration information. Thus,multiple beams for communications between the base station and theterminal may be configured.

FIG. 9 is a timing diagram illustrating a fourth embodiment of amulti-beam recovery procedure in a communication system.

Referring to FIG. 9 , when the beam failure recovery procedure iscompleted, the base station may transmit information requestingreporting of multi-beam configuration information to the terminalthrough a downlink channel (e.g., PDSCH). The information requestingreporting of multi-beam configuration information may be included in aMAC CE. That is, the information requesting reporting of multi-beamconfiguration information may be transmitted through the MAC CE insteadof an RRC message. Resource allocation information of the downlinkchannel for transmission of the information requesting reporting ofmulti-beam configuration information may be transmitted through the beamfailure recovery response signal in the beam failure recovery procedure.

The terminal may receive the information requesting reporting ofmulti-beam configuration information from the base station, and maytransmit multi-beam configuration information (e.g., indexes and/orqualities of configurable beams) through an uplink channel (e.g., PUCCHor PUSCH) to the base station. Resource allocation informationindicating the uplink channel for transmission of the multi-beamconfiguration information may be received together with the informationrequesting reporting of multi-beam configuration information.

The base station may receive the multi-beam configuration informationfrom the terminal, and may determine that multiple beams can beconfigured based on the multi-beam configuration information. Thus,multiple beams for communications between the base station and theterminal may be configured.

FIG. 10 is a timing diagram illustrating a fifth embodiment of amulti-beam recovery procedure in a communication system.

Referring to FIG. 10 , in the new candidate beam identification step, aplurality of beams (e.g., beam #1 and beam #2) may be configured as newcandidate beams. In this case, the terminal may transmit a beam failurerecovery request signal to the base station using each of the newcandidate beams (e.g., beam #1 and beam #2). The base station mayreceive the beam failure recovery request signal through the beam #1 andmay transmit a beam failure recovery response signal in response to thebeam failure recovery request signal by using the beam #1. The basestation may receive the beam failure recovery request signal through thebeam #2 and may transmit a beam failure recovery response signal inresponse to the beam failure recovery request signal by using the beam#2. The terminal may receive the beam failure recovery response signalthrough the beam #1 and the beam #2, respectively. Thus, multiple beams(e.g., beam #1 and beam #2) may be recovered through the beam failurerecovery procedure.

Here, the transmission and reception procedure of the beam failurerecovery request and response signals through the beam #1 may beperformed independently of the transmission and reception procedure ofthe beam failure recovery request and response signals through the beam#2. A PRACH resource for the beam #1 may be configured independently ofa PRACH resource for the beam #2. Each of the recovery timer and themaximum number of recovery requests for the beam #1 may be configuredindependently of each the recovery timer and the maximum number ofrecovery requests for the beam #2.

FIG. 11 is a timing diagram illustrating a sixth embodiment of amulti-beam recovery procedure in a communication system.

Referring to FIG. 11 , when the number of beams to be configured as newcandidate beams is not expected to be large, the number of combinationsof beams configurable as multiple beams may not be large. For example,when beams #1 to #3 are expected to be configured as new candidatebeams, the beam combinations may be configured as shown in Table 1below.

TABLE 1 Beams included in a beam combination Beam combination #1 Beam #1and Beam #2 Beam combination #2 Beam #1 and Beam #3 Beam combination #3Beam #2 and Beam #3

The base station may configure a PRACH resource (e.g., time-frequencyresource and preamble ID of PRACH) for each beam combination. Forexample, the PRACH resource for the beam combination #1 may be differentfrom the PRACH resource for the beam combination #2 or the beamcombination #3. The base station may transmit information on the PRACHresource for each beam combination to the terminal through an RRCmessage. The terminal may confirm the information on the PRACH resourcefor each beam combination by receiving the RRC message.

When the beam #1 and the beam #2 are determined as new candidate beamsin the new candidate beam identification step, the terminal may transmita beam failure recovery request signal to the base station through aPRACH configured for the beam combination #1. The beam failure recoveryrequest signal may be transmitted using a beam determined according to apreconfigured rule among the multiple beams. The preconfigured rule maybe one of the following rules.

-   -   Rule #1: The beam failure recovery request message is        transmitted through a beam having a low index among the multiple        beams.    -   Rule #2: The beam failure recovery request message is        transmitted through a beam having a high index among the        multiple beams.    -   Rule #3: The beam failure recovery request message is        transmitted through a beam having a low RSRP among the multiple        beams.    -   Rule #4: The beam failure recovery request message is        transmitted through a beam having a high RSRP among the multiple        beams.

The base station may receive the beam failure recovery request signalfrom the terminal, and identify a beam combination (e.g., beamcombination #1) in which multiple beams can be configured based on thePRACH resource through which the beam failure recovery request signal isreceived. The base station may transmit a beam failure recovery responsesignal to the terminal, which is a response to the beam failure recoveryrequest signal. The beam used for transmission of the beam failurerecovery response signal may be the same as the beam used for receptionof the beam failure recovery request signal. The terminal may receivethe beam failure recovery response signal from the base station. Whenthe beam failure recovery procedure is completed, the multiple beams(e.g., beam #1 and beam #2) may be configured between the base stationand the terminal.

The embodiments of the present disclosure may be implemented as programinstructions executable by a variety of computers and recorded on acomputer readable medium. The computer readable medium may include aprogram instruction, a data file, a data structure, or a combinationthereof. The program instructions recorded on the computer readablemedium may be designed and configured specifically for the presentdisclosure or can be publicly known and available to those who areskilled in the field of computer software.

Examples of the computer readable medium may include a hardware devicesuch as ROM, RAM, and flash memory, which are specifically configured tostore and execute the program instructions. Examples of the programinstructions include machine codes made by, for example, a compiler, aswell as high-level language codes executable by a computer, using aninterpreter. The above exemplary hardware device can be configured tooperate as at least one software module in order to perform theembodiments of the present disclosure, and vice versa.

While the embodiments of the present disclosure and their advantageshave been described in detail, it should be understood that variouschanges, substitutions and alterations may be made herein withoutdeparting from the scope of the present disclosure.

What is claimed is:
 1. A terminal in a communication system, comprising:a processor, wherein the processor causes the terminal to: search for aplurality of candidate beams when a beam failure is detected; transmit abeam failure recovery request signal to a base station using a beam #1among the plurality of candidate beams; receive from the base station abeam failure recovery response signal that is a response to the beamfailure recovery request signal through the beam #1; and when a beamfailure recovery procedure for the beam #1 is succeed, perform amulti-beam configuration procedure using the beam #1 which is a beamrecovered by the beam failure recovery procedure, wherein, when themulti-beam configuration procedure is performed, the processor causesthe terminal to: transmit a scheduling request (SR) to the base stationusing the beam #1, the SR requesting a resource for transmission ofmulti-beam configuration information including index(es) of one or morebeams excluding the beam #1, which is the beam recovered by the beamfailure recovery procedure, among the plurality of candidate beams;receive an uplink (UL) grant from the base station using the beam #1,the uplink grant being a response to the SR; and transmit the multi-beamconfiguration information to the base station using a resource indicatedby the UL grant and the beam #1.
 2. The terminal according to claim 1,wherein the beam failure is detected based on a measurement result of afirst reference signal configured by the base station, and the firstreference signal has a quasi-co-location (QCL) relationship with ademodulation reference signal (DM-RS) of a control channel between thebase station and the terminal.
 3. The terminal according to claim 1,wherein the beam failure is determined to be detected when error ratesof all beams used for transmitting and receiving a control channelbetween the terminal and the base station are larger than apreconfigured threshold, and when a part of all the beams are receivedwithin a monitoring period and error rates of the part of all the beamsare larger than the preconfigured threshold, a physical (PHY) entity ofthe terminal transmits to a medium access control (MAC) entity of theterminal a flag indicating resetting of a timer without increasing acounter indicating a number of occurrences of beam failure instances. 4.The terminal according to claim 1, wherein the beam failure isdetermined to be detected when error rates of all beams used fortransmitting and receiving a control channel between the terminal andthe base station are larger than a preconfigured threshold, and when atleast one beam among all the beams is received within a monitoringperiod, a PHY entity of the terminal transmits to a MAC entity of theterminal information indicating a beam(s) having an error rate largerthan the preconfigured threshold.
 5. The terminal according to claim 1,wherein the plurality of candidate beams are searched based onmeasurement results of second reference signals configured by the basestation, and each of the second reference signals has a QCL relationshipwith a DM-RS of a control channel between the base station and theterminal.
 6. The terminal according to claim 5, wherein the beam failurerecovery request signal is transmitted through a physical random accesschannel (PRACH) associated with the second reference signal.
 7. Theterminal according to claim 1, wherein the beam failure recoveryresponse signal is received through a control resource set (CORESET)configured by the base station.
 8. The terminal according to claim 1,wherein the multi-beam configuration information further includes atleast one of information indicating that multiple beams can beconfigured and information of qualities of the one or more beams.
 9. Theterminal according to claim 1, wherein the multi-beam configurationinformation is transmitted to the base station through a MAC controlelement (CE).
 10. A terminal in a communication system, comprising: aprocessor, wherein the processor causes the terminal to: search for aplurality of candidate beams when a beam failure is detected; transmit abeam failure recovery request signal to a base station using a beam #1among the plurality of candidate beams; receive from the base station abeam failure recovery response signal including an uplink (UL) grantthrough the beam #1; and when a beam failure recovery procedure for thebeam #1 is succeed, perform a multi-beam configuration procedure usingthe beam #1 which is a beam recovered by the beam failure recoveryprocedure, wherein, when the multi-beam configuration procedure isperformed, the processor causes the terminal to: transmit to the basestation multi-beam configuration information including index(es) of oneor more beams excluding the beam #1 among the plurality of candidatebeams, by using a resource indicated by the UL grant and the beam #1.11. The terminal according to claim 10, wherein the processor furthercauses the terminal to, after receiving the beam failure recoveryresponse signal, receive a medium access control (MAC) control element(CE) requesting reporting of the multi-beam configuration informationfrom the base station, wherein the multi-beam configuration informationis transmitted when the MAC CE is received.
 12. The terminal accordingto claim 10, wherein the beam failure is detected based on a measurementresult of a first reference signal configured by the base station, andthe first reference signal has a quasi-co-location (QCL) relationshipwith a demodulation reference signal (DM-RS) of a control channelbetween the base station and the terminal.
 13. The terminal according toclaim 10, wherein the plurality of candidate beams are searched based onmeasurement results of second reference signals configured by the basestation, and each of the second reference signals has a QCL relationshipwith a DM-RS of a control channel between the base station and theterminal.
 14. The terminal according to claim 10, wherein the multi-beamconfiguration information further includes at least one of informationindicating that multiple beams can be configured and information ofqualities of the one or more beams.
 15. The terminal according to claim10, wherein the multi-beam configuration information is transmitted tothe base station together with a buffer status report (BSR).
 16. Aterminal in a communication system, comprising: a processor, wherein theprocessor causes the terminal to: search for a plurality of candidatebeams when a beam failure is detected; transmit a first beam failurerecovery request signal to a base station using a beam #1 among theplurality of candidate beams; receive from the base station a first beamfailure recovery response signal that is a response to the first beamfailure recovery request signal through the beam #1; when a beam failurerecovery procedure for the beam #1 is succeed, perform a multi-beamconfiguration procedure for a beam #2 among the plurality of candidatebeams; and perform communication with the base station using multi-beamincluding the beam #1 and the beam #2, wherein, when the multi-beamconfiguration procedure is performed, the processor causes the terminalto: transmit a second beam failure recovery request signal to the basestation using the beam #2 among the plurality of candidate beams; andreceive from the base station a second beam failure recovery responsesignal that is a response to the second beam failure recovery requestsignal through the beam #2.
 17. The terminal according to claim 16,wherein the beam #1 is searched based on a measurement result of a firstreference signal configured by the base station, the first referencesignal has a quasi-co-location (QCL) relationship with a demodulationreference signal (DM-RS) for a control channel between the base stationand the terminal, and the first beam failure recovery request signal istransmitted through a first physical random access channel (PRACH)associated with the first reference signal.
 18. The terminal accordingto claim 17, wherein the first PRACH is configured for the beam #1 and abeam #3 among the plurality of candidate beams, and the first beamfailure recovery request signal is a signal for requesting recovery ofthe beam #1 and the beam #3 which are associated with the first PRACH.19. The terminal according to claim 17, wherein the first PRACH isconfigured independently of a second PRACH through which the second beamfailure recovery request signal.
 20. The terminal according to claim 16,wherein configuration information of a PRACH used for recovery of beamfailures of two or more beams among the plurality of candidate beams isreceived from the base station through a radio resource control (RRC)message.